Surround sound processing, in general, is a technique wherein a stereophonic pair of signals from a source such as prerecorded audio or live transmissions of audio signals, with or without video, is processed to yield a set of signals for the purpose of feeding several loudspeakers placed around the listening area, so as to give an impression of spatially surrounding the listener with the sounds, particularly any ambience, and/or broadening the sound field to wrap around the listener.
Typically, the signals are generated by surround sound processors, which may be of many types, and the source signals may either incorporate positional encoding by means of specific phase differences and amplitude ratios, or may be unencoded. Surround sound processors may be divided into two main classes, fixed matrix or passive, and variable matrix or active types.
In the former, the various loudspeaker feed signals are derived by judicious mixing or matrixing of the pair of stereophonic signals in different proportions and different relative phases, with matrixing coefficients which remain fixed in time, and have no relationship to the instantaneous signal information content.
In the latter, in addition to generating a fixed matrix for each loudspeaker feed, a number of control signals are derived from the input signal pair according to the relative amplitudes and phases of these signals, and a number of cancellation signals are generated by means of variable gain elements whose gains are controlled by the various control signals, the cancellation signals being applied to the loudspeaker feed matrix so as to reduce crosstalk between the loudspeaker feed signals and increase separation of the predominant sound in the direction in which it is desired to be heard. Thus the signals are processed with a variable matrix, which is responsive to the signal information content.
Surround processors of the variable matrix type described have been the subject of several inventions by Fosgate, specifically including those described in U.S. Pat. No. 4,932,059 and copending U.S. patent application Ser. No. 533,091 entitled "Surround Processor".
The proper placement and types of loudspeakers for such surround sound processors have not usually been well defined, although in general the array of loudspeakers is assumed to be placed in specific positions relative to the listener--usually at least four loudspeakers are placed in a square or rectangular array, one at each corner, with the listener at or near the center of the array, and additional speakers may be placed in the central positions of the front, rear or sides of the rectangle. Other speakers for special purposes, such as subwoofers for deep bass, may also be positioned to surround the listener in some such way, if desired.
Some newer surround sound systems, such as that known as "Ambisonics" due to Gerzon, have attempted to control the sound field by such means as a "layout" control which is adjusted to suit the approximate size and shape of the rectangular array of four loudspeakers or various other possible arrangements, e.g. of six loudspeakers. The Lucasfilm, Ltd. THX system also specifies an array of loudspeakers having certain characteristics and placed in a certain arrangement in the listening room. Reference is made to the specification for the THX Home Theater Surround prepared by Lucasfilm, Ltd., for example.
In the latter system, intended for the reproduction of surround-encoded movie sound tracks, the proprietors intend that the surround sound effects be diffused in the rear of the room, this being achieved by several methods in combination, including the provision of rear loudspeakers having dipole radiation characteristics such that the rearward radiation lobe thereof is out of phase with the frontward radiation lobe in the midrange and upper frequencies. However, for widely spread surround sound presentation of music from a stereophonic source, for example, it is desirable that the sound field around the sides and rear of the room remain focused and in phase, so that imaging of sounds is good all around the room.
Accordingly, there is a need for the development of surround sound systems capable of generating a set of signals for application to an array such as to meet the general THX specifications, as well as being switchable to other modes for presentation of musical or other stereophonic or monophonic program material as desired by the listener, so as to give either a more focused or a more diffused spatial sound field. There is further a need for the provision of appropriate types of loudspeaker arrangements for use in conjunction with such surround processor systems.